bpf: Attachment verification for BPF_MODIFY_RETURN

- Allow BPF_MODIFY_RETURN attachment only to functions that are:

    * Whitelisted for error injection by checking
      within_error_injection_list. Similar discussions happened for the
      bpf_override_return helper.

    * security hooks, this is expected to be cleaned up with the LSM
      changes after the KRSI patches introduce the LSM_HOOK macro:

        https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/

- The attachment is currently limited to functions that return an int.
  This can be extended later other types (e.g. PTR).

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200304191853.1529-5-kpsingh@chromium.org

1 files changed, 20 insertions, 8 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 30841fb8b3c0..50080add2ab9 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3710,14 +3710,26 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		nr_args--;
 	}
 
-	if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||
-	     prog->expected_attach_type == BPF_MODIFY_RETURN) &&
-	    arg == nr_args) {
-		if (!t)
-			/* Default prog with 5 args. 6th arg is retval. */
-			return true;
-		/* function return type */
-		t = btf_type_by_id(btf, t->type);
+	if (arg == nr_args) {
+		if (prog->expected_attach_type == BPF_TRACE_FEXIT) {
+			if (!t)
+				return true;
+			t = btf_type_by_id(btf, t->type);
+		} else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
+			/* For now the BPF_MODIFY_RETURN can only be attached to
+			 * functions that return an int.
+			 */
+			if (!t)
+				return false;
+
+			t = btf_type_skip_modifiers(btf, t->type, NULL);
+			if (!btf_type_is_int(t)) {
+				bpf_log(log,
+					"ret type %s not allowed for fmod_ret\n",
+					btf_kind_str[BTF_INFO_KIND(t->info)]);
+				return false;
+			}
+		}
 	} else if (arg >= nr_args) {
 		bpf_log(log, "func '%s' doesn't have %d-th argument\n",
 			tname, arg + 1);